In the process of stamping, in particular dies are applied with pressurizing force of a stamping press, reaction force due to deformation resistance of work and so forth, and cause elastic deformation. The elastic deformation of the dies is referred to as die strain. Since any stamping failure such as fracture in the process of stamping may vary the amount of die strain to be produced, so that it is very important to measure the amount of die strain. The die strain may, however, vary depending not only on the stamping failure, but also on external disturbance factors such as variation in material strength of works, changes in temperature and so forth. It is, therefore, expected to develop a technique of more effectively predicting stamping failure phenomena based on information of the amount of die strain.
As a device for measuring die strain, Patent Document 1 discloses a boat-form correction device for a press brake which bends a workpiece between a punch attached to an upper beam and a die attached to a lower beam, moved to join with and disjoin from each other. The boat-form correction device has a plurality of upper beam strain sensors for detecting deflection of the upper beam provided in the longitudinal direction of the upper beam; a plurality of lower beam strain sensors for detecting deflection of the lower beam provided in the longitudinal direction of the lower beam; a plurality of actuators disposed between the lower beam and a lower die, or between the upper beam and an upper die, in a distributed manner in the direction of line of bending so as to apply pressurizing force to the lower die or the upper die; and a control unit which interrupts descending of the upper beam at a point of time between the start and end of pressurizing, fetches outputs of detection by the upper beam strain sensors and the lower beam strain sensors during the interruption, calculates the amount of deflection of the upper beam and the lower beam based on the outputs of detection, controls operations of the plurality of actuators based on the calculated results so as to adjust the amount of deflection of the upper beam and lower beam to appropriate values, and then restarts control of the pressurizing.
As a device having a model for predicting deformation of dies, Patent Document 2 discloses a stamping die which has a load detection unit 4; a stroke detection unit 5; a detection unit 6 for the number of times of stamping; a detection unit 7 for die temperature; a deformation prediction model which is composed of any one of, or a plurality of models selected from a wear model 9 of die, thermal deformation model 10 of die, load-deformation model 11 of die, thermal deformation model 12 of work, and a spring back model 13 of work; a multi-variable control signal generator 14; and an actuator 15 such as a piezoelectric element, for deforming the inner wall of a cavity 3, aiming at providing a stamping dies capable of precisely and automatically controlling dimension and geometry of products, for the purpose of simplifying the process by promoting production of near-net-shape stamped products in stamping. Note that the reference numerals attached herein are those used in Patent Document 2.
As a device for measuring strain of dies, Patent Document 3 discloses an invention configured as described below, aiming at providing dies capable of detecting angle of bending of works and strain of works, and a strain sensor unit used therefor. In a strain-producible region where strain of a work W occurs in the process of working, and in a plane normal to a work-supporting surface 3U on which a main unit 3 of die supports the work, the main unit 3 of die is provided with strain sensors 9 buried therein at two or more positions close to, and distant from, the position of working of the work W. The plurality of strain sensors 9 are disposed in a traversing manner between the top surface 3U of a main unit of die 3 and groove-forming surface 5F having a bend groove 5 formed therein. A strain sensor unit has the sensors for detecting strain at a plurality of positions on the base 13 composed of an insulating material. The base 13 is configured to have sensor-attachment surfaces 21A, 21B having the sensors 9 attached thereto, and bonding surfaces allowing thereon the base 13 to be bonded to the inner surface of a fitting hole in an integrated manner, so as to form a gap between each sensor attachment surface and the inner surface of the fitting hole, when the base is fitted to the fitting hole. The reference numerals attached herein are those used in Patent Document 3.
As a device for measuring strain of dies, Patent Document 4 discloses an invention configured as described below, aiming at providing dies capable of increasing pressurizing force depending on the depth (stroke) of works in a V-shape groove. A die 1, used for bending a sheet-form work W into V-shape, has an inclined surface 5 composing a V-groove 3, formed into convex curved surfaces 5U, 5L, wherein a curvature DR2 of the lower portion of the curved surface is larger than a curvature DR1 of the upper portion of the curved surface, so as to allow the curved surfaces 5U, 5L to contact with a work W in a mode of rolling contact in the process of bending. The die 1, used for bending a sheet-form work W into V-shape, is alternatively configured to have an inclined surface 5 for forming a V-groove 3, formed into a convex curved surface which is in contact with an oval. The die 1 has a sensor 7 for detecting deformation, at a position close to the inclined surface 5. The reference numerals attached herein are those used in Patent Document 4.
As a device for measuring strain of dies, Patent Document 5 discloses an invention configured as described below, aiming at providing a method and a device for detecting angle of bending of works by detecting strain of dies. In the process of bending of a sheet-form work W, a deformation pattern of a die 5 is measured using strain sensors S1 to S4 provided to the die 5, a predictive equation is determined by comparing the measured strain pattern with a plurality of strain patterns preliminarily stored in a database 15, friction coefficient is found from the thus-determined pattern predictive equation, and an angle of bending of the work is calculated based on the value detected by the strain sensors, using the pattern predictive equation. The strain sensors for detecting strain of die in the process of bending of a work are provided at a plurality of positions. A control unit 9 has a database 15, a predictive equation determining unit 17 which determines a pattern predictive equation by comparing detected strain of the die 5 with strain patterns stored in the database 15, and a calculation unit 21 which determines friction coefficient using the pattern predictive equation. The reference numerals attached herein are those used in Patent Document 5.
As a device for measuring strain of dies, Patent Document 6 describes an invention configured as described below, aiming at ensuring desirable stamping. An arithmetic unit 104 for conditional setting compares actual values of material characteristics sent from a material characteristic data providing unit 101 through a network 105 with standard values, and corrects conditions of forming such as sliding speed and blank holding force depending on results of the comparison. A control unit 300 controls a stamping press 102 so as to start stamping of a work 300 according to the corrected conditions of forming. In this way, cracks and wrinkles are prevented as possible from generating in the process of stamping of a work 300, so as to obtain acceptable products having a geometry identical as possible. The reference numerals attached herein are those used in Patent Document 6.
Patent Document 7 discloses a stamping press for thin sheet, having a punch, a die, a blank holding die, a frictional force measurement unit attached between the die and the blank holding die, and a blank holding force adjustment unit. Frictional force is directly measured by the frictional force measurement unit, and blank holding force is further controlled by a blank holding force adjustment unit so as to adjust the frictional force to a predetermined value. The invention aims at applying an appropriate frictional force irrespective of variable factors such as lubricating performance between the die and a work, surface property and so forth, and thereby constantly providing acceptable products.
Patent Document 8 discloses an invention configured as described below, aiming at providing a stamping press capable of controlling die strain in the process of stamping. The invention includes a punch; a die which moves relative to the punch; a strain measuring unit provided inside a target to be controlled which is at least either one of the punch and the die, so as to measure the amount of strain of the target to be controlled produced as the stamping proceeds; and a strain control unit provided to the target to be controlled so as to control the amount of strain produced as the stamping proceeds. The strain control unit controls the amount of drive of the target to be controlled so as to adjust the amount of strain measured by the strain measuring unit within a predetermined range in the process of forming.
The present inventors disclosed a stamping press in Non-Patent Document 1, which has a built-in piezoelectric element (die friction sensor) for measuring compressive and tensile strain in the orthogonal directions, provided in the vicinity of a shoulder portion of a die, and made clear that the geometry of the works, such as spring-back and tilting, may be predictable from information given by the die friction sensor.